docs: def _get_data_from_csv_files(self):
      """Get data from input csv files."""
  
      all_df = []
      for file_name in self._input_csv_files:
        with _util.open_local_or_gcs(file_name, mode='r') as f:
          all_df.append(pd.read_csv(f, names=self._headers))
      df = pd.concat(all_df, ignore_index=True)
      return df